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Chapter 2

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Simon Fraser University
Biomedical Physio & Kines
BPK 140
Michael Walsh

1 KIN 140 Section 2 Dr Mike Walsh Life comes from other life and disease comes from germs (and a few other things) I. Infectious Diseases Adisease is any deviation from normal functioning.An infectious disease is a disease in which a pathogen is present and may multiply. Of course, this does not include, for example, mercury poisoning. A. Pathogens Apathogen is an agent that produces disease. Types of pathogens include the following: 1. Bacteria These are single-celled organisms with their own DNAand no membrane bound organelles. About 1500 are well classified and about 100 of these are pathogens. Some bacteria are very helpful to aid us in such things as digestion of food in our intestines (remember the intestines are outside of us). The inside of our body is virtually sterile. Bacteria within our body are almost always pathogenic and attacked by our immune system. Those bacteria that survive the body’s defenses multiply and cause infection. The infection can occur in virtually any tissue in our body. Note: the gastrointestinal tract is not in our body Lets keep bacteria in some perspective. In terms of mass, there are more bacteria on this planet than plants and animals combined) Examples of Bacterial Infection Tuberculosis. The number one infectious disease in the world Lyme’s Disease Meningitis (also other causes) Strep Throat Tetanus. Not infectious. 2. Viruses 2 These are the smallest ‘living’pathogens. They consist primarily of small amounts of genetic material (DNAor RNA) covered by a protein coat. They are a borderline life form because they require other cells to reproduce. Unlike bacteria, they do not have their own life cycle. Viruses can reproduce in a variety of human cell types (and other cell types such as bacteria or plants). Examples of Viral Infection Flu. The common cold is caused by the influenza virus in its various forms. Polio. The poliovirus reproduces in motor neurons which cannot be replaced and thus the consequence of such an infection is severe. Herpes. The herpes virus can be in one’s body for life. When conditions are right, the formally dormant virus begins to reproduce (an opportunistic virus). Hepatitis. Mononucleosis Warts human papilloma virus 3. Fungi These are primitive plant forms that may be unicellular (yeast) or multicellular (molds). Fortunately fungal infections are restricted to the skin and only about 50 of the thousands of known fungi cause disease in humans. Examples of Fungal Infection Candida albicans (candidiasis). This is a yeast infection in a woman’s vagina. This normal resident can cause problems when it suddenly multiplies. Athlete’s Foot. 4. Protozoa These are single-celled animals. Many of these diseases are recurrent as the pathogen alternates between inactive and active phases. Examples of Protozoa Infection Giardia (Beaver fever). This can arise for drinking untreated water resulting in diarrhea, nausea, and abdominal cramps. Malaria. 3 5. Parasitic Worms. These include tapeworms, ringworms, and leeches. They generally arise from contaminated food and drink. 6. Prions These are protein particles only. Prions are proteins found in mammalian cell walls. They occur more frequently in the central nervous system. We do not know their normal function. But aberrant prions can cause mad cow disease (bovine spongiform encephalopathy). B. Chain of Infection 1. Pathogen The pathogen can be any disease-causing agent, infectious or otherwise and include those discussed above. 2. Reservoir The reservoir can be human, animal, inanimate, or environmental. In the reservoir, the pathogen can either be stable or multiply. If it multiplies in a living host, symptoms of the disease will develop. 3. Portal of Exit The pathogen can leave a human host by many ‘portals’including all bodily fluids. Saliva for mumps, mucous membranes for sexually transmitted diseases (STDs), blood for HIV, feces for intestinal infections, and throat and nose mucous for colds and flu. 4. Mode of Transmission Acommunicable disease is one that can be transmitted from one host to another (a contagious disease is one that is spread easily). The pathogen must travel from the reservoir to a susceptible host. This can be done by 2 classification mechanisms. a. Direct Transmission 4 Human to human transmission can be done via body surface contact (kissing, shaking hands) Droplet spread. Inhalation of someone’s breath (especially a sneeze) Fecal-oral spread. Feces on the hands brought into contact with the mouth. b. Indirect T
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